In order to give a pleasing appearance to the base material of a watch case, e.g. of steel this will generally be coated over at least its visible portions by a galvanic layer for instance. It is thus that there may be obtained in a known manner a chrome or gold plated case. It is known that these coatings are of small wear resistance and almost no abrasion resistance.
To overcome this difficulty, the French patent document No. 2 425 210 (corresponding to U.S. Pat. No. 4,252,862) suggests coating the object with a layer of titanium nitride (TiN) which, at the same time as being hard and resistant to abrasion, exhibits an external surface of a colour approximating that of gold. However, in the cited document, there has been foreseen solely the employment of brass or of stainless steel as the basic body of the object. These metals generally exhibit a matt surface and it will be noted that following application of TiN, the surface of the object is extremely sensitive to finger marks which leave indelible prints. To overcome this difficulty the cited document suggested covering the layer of TiN by a layer of gold or gold alloy which in addition would further improve the resemblance of the colour to that of gold. According to the explanations given in this same document, the layer of TiN is applied by physical vapour deposition (PVD).
It is found that if one applies the teaching of the cited document to a case body of hard metal, the hardness of which exceeds Vickers 600 HV.sub.1 (for example at least temperated steel DIN 1.4125 or Stellite) but being preferably in the neighbourhood of 1400 HV.sub.5 (for instance tungsten carbide), it will be noted that the fingers no longer leave the undesired prints since the body of the case exhibits a polished surface. It will thus no longer be necessary to apply a supplementary layer of gold. At the same time, it is noted that the adhesion of the layer of TiN on the hard metal substrate is poor. This is due in particular to the fact that there exist heavy strains between the layer of TiN and its support, such strains being evidenced during measurement of the Vickers hardness where one will note frequent chipping of the layer of TiN at the time that the measuring punch in the form of a lozenge is applied. This default of poor adhesion may also be evidenced by a file test (file having a hardness of 45 Rockwell with 30 lines per centimeter).
To minimize the strains as mentioned hereinabove, it is well known to interpose a layer of titanium (Ti) between the body of the hard metal case and the layer of TiN. This will result in a better adhesion of the layer of TiN since the strains as indicated are absorbed by the soft undercoating of titanium. At the same time, apart from the fact that the layer of TiN may still be removed quite easily because of the low hardness of the titanium layer (&lt;300 HV.sub.1), the proposed solution loses a great deal of its interest since the scratch proof nature of the case body disappears owing to the presence of titanium as an undercoating. To overcome this difficulty, one could reduce the thickness of the titanium layer to less than 0.2 .mu.m. Experience has shown however that such a thin layer is very difficult to obtain. In any event and as has been mentioned, the coating will not resist the file test.
To conclude what has just been said, all methods known to this day and comprising the deposition of a TiN layer by physical vapour deposition (PVD) onto a hard metal are unsuitable to satisfy the scratchproof criterion.
In following through the investigations in this domain, the applicant sought to deposit a layer of titanium nitride onto a hard metal no longer by the PVD procedure, but by chemical vapour deposition (CVD). It was then possible to obtain a layer of TiN very adherent to the case body (chemical bond) and at the same time satisfying the file test as mentioned hereinabove. However such layer exhibited not only a colour remote from that of gold, but further a lack of evenness as to the coloration obtained.